1. Field of the Invention
The present invention relates to a solid electrolytic capacitor using conductive macromolecules as a solid electrolyte, and more particularly, to a solid electrolytic capacitor which is encased in a synthetic resin molded casing, hereinafter referred to as the "casing". The present invention also relates to a method for producing solid electrolytic capacitors of this kind.
2. Description of the Prior Art
In line with the recent trend in which electronic circuits are digitized and have high frequencies, capacitors used therefor are required to have high reliability, small-size, large capacitance and excellent high frequency characteristics. Solid electrolytic capacitors, which are inherently small-sized and have large capacitance, have been improved by using solid electrolytes of newly developed conductive macromolecules. As a result, they have a high electroconductivity of conductive macromolecules of about 10.sup.4 s/m as compared with that of manganese dioxide (10.sup.0 s/m or TCNQ salt (10.sup.20 s/m) and high thermal stability. The improved solid electrolyte capacitors have desirable characteristics such as stable frequency characteristics of impedance and thermal characteristics within a wide range.
However, it is necessary to reduce inductance caused by external terminals of the capacitor in accordance with increases in frequency. It is also required to occupy a minimum space on a circuit substrate in order to reduce the size of the electronic devices. Furthermore, the consistent reliability of capacitors are essential so as not to spoil the sophisticated, long-term electronic devices. Therefore, chip type elements which are small-sized, highly reliable, and flexible in mounting, that is, can be mounted in a desired position without occupying a large space, such as are capable of being, vertically stacked or horizontally arranged, are desirable. Conventional elements are disadvantageous in that inductance is likely to become large and mounting flexibility is lacking.
Referring to FIGS. 17 and 18, a conventional chip type solid electrolytic capacitor will be described, as follows:
The solid electrolyte capacitor uses conductive macromolecules as solid electrolyte. The type shown in FIG. 17(a) is covered with positive electrode foils, and the type shown in FIG. 17(b) is provided with plain positive electrode foils, and FIG. 18 shows a further type of low chip solid electrolyte capacitor element. The positive foils are etched to form an oxide film as dielectric. Then a conductive macromolecular layer, a graphite layer, and a silver paint layer are formed on the portion except the lead line of the positive electrode. A positive electrode terminal 3 and a negative electrode terminal 4 are connected in parallel with the capacitor elements 1 and 2, respectively. Then, the capacitor elements are encased in casing such as a transfer mold or a pot. The whole body is flat, with a wide bottom and a short height as shown in FIG. 18.
The conventional types shown in FIGS. 17(a) and 17(b), and FIG. 18 are disadvantageous in that when they are densely mounted on a circuit substrate, they horizontally expand and occupy a large space of the substrate. In addition, the positive electrode terminal 3 and negative electrode terminal 4, which are both external terminals, are opposed to each other so that a current is difficult to efficiently pass through the capacitor elements 1 and 2 through the surfaces of a land of the print wirings. As a result, the capacitance of the capacitor is not fully utilized, and inductance cannot be minimized.
Furthermore, the conventional solid electrolyte capacitors are disadvantageous in that they are difficult to be mounted without losing reliability. When the solid electrolyte capacitors are to be coated with synthetic resins as described above, they are placed at a high temperature such as 250.degree. C. This high temperature unfavorably affects the quality of the capacitors; such as the deterioration of characteristics. The reasons are as follows:
(1) In order to improve solderability, the surface of the frame functioning as the terminal is plated with tin or any other metal which has a relatively low melting point. The metal is likely to melt in the mounting process to cause cavities between the external synthetic resins and the frame.
(2) Because of the larger coefficient of expansion of the synthetic resin used in mounting than that of the metal of the frame, the frame and the synthetic resin are likely to separate from each other, thereby causing a gap therebetween.
The laminated type shown in FIG. 17(b), which has a large capacitance for its relatively small size, is nevertheless disadvantageous in that the connection between the capacitor elements, and between the capacitor elements and the frame are not reliable.
Japanese Laid-Open Patent Publication No. 63-239917 discloses another laminated type as illustrated in FIGS. 19 and 20 which uses conductive macromolecules as a solid electrolyte. This type includes an etched aluminum foil strip 6 having several projections 7 along one of the edges. Each projection 7 is divided into a negative electrode portion and a positive electrode portion by a photoresist band 8. The negative electrode is composed of an aluminum oxide film layer 9 as dielectric, a polymer layer 10 of conductive macromolecules made of pyrrole as heterocyclic compound polymer layer, a graphite layer 11 as conductive layer for the leading terminal, and a silver paste layer 12.
A plurality of laminated units 13 each constructed in this way are stacked as shown in FIG. 21(a), wherein the projections 7 overlay one another to which pressure is applied at high temperatures. The silver paste layer 12 is half dried, and a part 14 of the laminated unit 13 is joined to a part of the other laminated unit 13 so as to obtain a union of the two units 13, wherein portions 15 (portions marked "x" in FIG. 20) of the etched aluminum foil strips 6 are welded to each other. In this way, a solid electrolyte capacitor 16 is formed.
Subsequently, the half dried silver paste layer 12 is finally dried so as to enable the negative electrode 14 to join to the surface of a planar negative terminal 17 under pressure. A planar positive electrode terminal 18 is joined to the positive electrode 15 by spot welding or ultrasonic welding to form a capacitor element which is finally coated with synthetic resin.
In addition to the disadvantages pointed out above, the known solid electrolytic capacitor are disadvantageously lacking in the reliability of the connection of the positive electrode. Valve metals such as aluminum are liable to oxidation in the atmosphere and oxide films are likely to be formed on the surface. When such oxide films are layered, it is difficult for spot welding or ultra sonic welding to penetrate all the oxide film layers and effect a firm welding joint therethrough.
When an apparently successful welding joint is made, contact resistance is large thereby causing the joined elements to separate from each other when the capacitors are used. This is because the welding energy only reaches the oxide film layers but does not spread through all the welds. When resistance welding process such as spot welding is applied, it often happens that other current than the welding current flows through the oxide film layers and breaks them and the conductive macromolecular film.